Stratum corneum, the outer layer of skin, is a multicellular membrane of flattened, metabolically active cells. In living animals, the membrane is dynamic, in that surface cells are lost through desquamation but replaced at an equivalent rate from the underlying epidermal cells to maintain a thickness of a constant number of cells in the stratum corneum. Alterations in the rate of desquamation can result in a thickened, less flexible corneum.
To function normally as a diffusion and mechanical barrier, the stratum corneum must be sufficiently strong and flexible to reversibly sustain large deformations without fracturing. This elasticity is a function of both the configuration of the keratinous fibrous components of the cells and the presence of plasticizers, principally water. When not properly plasticized, the membrane becomes less flexible, and surface cells fail to desquamate normally, producing scale formation and surface roughness. Dry skin conditions ranging in severity from mild chapping to ichthyosis and psoriasis have been shown to be associated with macromolecular and morphological defects involving insufficient plasticization which may be responsible for the lack of flexibility and associated scaling and roughness. Accordingly, effective treatment to relieve or prevent the symptoms of these dry skin conditions requires physically altering or restoring the plasticized conditions of the macromolecular keratinous material and promoting desquamation of the irreversibly altered surface cells.
A description of a related therapy for scaly skin appeared in a recent publication by E. Van Scott and R. Yu in Arch. Dermatology 110:586-590 (1974) and more recently in Van Scott et al. U.S. Pat. No. 3,879,537 issued Apr. 22, 1975. The therapy is for control of keratinization with alpha-hydroxy acids and related compounds. The composition and method of the present invention differ from Van Scott et al. in employing the aliphatic alcohols in combination with the acids. Thus, Van Scott et al. did not find or teach what we have discovered--that the combination of their acids with n-propanol or a C.sub.4 -C.sub.8 aliphatic alcohol is significantly more effective in clearing the scales and cracking of dry skin than an equivalent concentration of acid alone. The only alcohols which appear to be mentioned in the Van Scott patent are ethanol and isopropanol. The use of alcohols in accordance with the present invention permits the use of significantly lower concentrations of these acids than described in the Van Scott patent. Moreover, while all acids disclosed by Van Scott et al. are suitable for use in the compositions of the present invention, we have found that numerous other organic acids have beneficial value in our compositions.
Swanbeck U.S. Pat. No. 3,666,863 relates to a skin treating composition comprising an aqueous solution of urea (2 to 30%) and lactic acid (0.5 to 8%). Swanbeck suggests that although urea alone is useful in treating dry skin, its combination with the alpha hydroxy acid, lactic acid, is more effective. However, we have found in tests on heavily scaled elbows and knees that, at comparable concentrations, the combination of lactic acid and urea with n-butanol was significantly more effective than the combination of urea and lactic acid alone. Moreover, we have found that when we combine an alcohol with the acid and urea in accordance with our invention, we require substantially less acid and urea than the amounts preferred by Swanbeck.
The combination of an aliphatic alcohol and an organic acid would not be expected to be an effective treatment for dry skin conditions since aliphatic alcohols and organic acids are not highly hygroscopic; alcohols are considered to have a drying effect on the skin surface, and the combination does not form a water impermeable film to prevent water loss from the tissue. Moreover, structurally closely related alcohols to those employed in the compositions and method of the present invention do not afford these beneficial effects. Thus, ethanol and isopropanol, commonly used in cosmetic formulations, have been found to be ineffective when employed as the alcohol in the compositions and method of the present invention. Similarly, inorganic acids such as HCl and H.sub.2 SO.sub.4 are not nearly as effective as the presently claimed acids, nor are many organic acids such as formic or acetic acid, when employed as the acid in the composition of the present invention, illustrating that these beneficial effects are not a result merely of the acidic pH of the composition.
The composition and method of the present invention are not directed at primary underlying causes of faulty keratinization in the conditions discussed above, but rather at relieving the discomfort of associated scales and ensuing cracking and bleeding. The essential property of the present skin treating composition is that it promotes the loss of the altered surface corneum cells while it increases corneum flexibility so that further cracking is reduced, allowing any existing lesions to heat without further mechanical injury.